This invention relates to wind turbines.
Conventional horizontal axis wind turbines suffer from certain drawbacks, some of which are:
1. High Mass of Large Rotors:
The mass of a rotor increases as function of the diameter cubed, while the swept area only increases as a function of the diameter squared. The amount of wind captured, per unit rotor mass, is therefore inversely proportional to rotor diameter. The single large rotor captures less wind per unit mass than a plurality of smaller rotors sweeping an equivalent area would. Such a single, large, heavy rotor also mandates the use of a commensurately stronger drivetrain and tower to support its ponderous weight.
2. Slow Rotation Rate of Large Rotors:
Today""s windmills, with their single large, slowly turning rotor require either a specially built, slow-speed alternator or generator, or a transmission means providing ratio gearing, such as a gearbox, to bring the rotation rate up to a speed compatible with a generator. Either solution is complicated, expensive, and heavy, adding to the cost of the installation, as well as the strength required of the supporting tower.
For a given wind speed, the tip speed of similarly shaped rotors is substantially the same, regardless of diameter. The rotational rate is therefore inversely proportional to rotor diameter, meaning that a smaller rotor spins faster to maintain the same tip speed as a larger, more slowly rotating set of blades. Conventional generators and alternators typically require such a fast rotation rate for efficient operation. Small rotors, turning more quickly, can therefore often directly drive a substantially standard alternator or generator without ratio gearing, or a transmission. With smaller rotors, if a transmission is required, it need incorporate less ratio gearing, and may therefore be less substantial, since the rotational rate of a smaller rotor is faster to begin with.
3. Faster Rotation Delivers the Same Power at Lower Torque:
A given amount of power is delivered at lower torque by a faster rotating shaft, further reducing the required robustness, and therefore the cost and weight, of the drivetrain.
4. Low Power Output from Smaller Rotors in Prior Art:
Though smaller rotors are desirable from the standpoint of achieving a higher rotation rate, the amount of wind power available from the area swept by a smaller rotor is less than that of a larger rotor, being proportional to the diameter squared. Conventional windmills having a single small rotor therefore require high winds for useful amounts of energy to be generated.
Many schemes have been put forward in the prior art to mechanically harness a multiplicity of smaller rotors together to power a single load. None has proven to be simple and reliable enough to have enjoyed commercial success. Prior art designs utilizing a multiplicity of rotors coupled to a single shaft disposed these rotors closely together, and directly in line with the wind, and had no means for supplying fresh wind to each rotor, and therefore suffered from excessive wind shadow effects between rotors, making the redundancy of multiple rotors largely ineffective, non-advantageous, and indeed, burdensome and unworkable.
5. A dedicated azimuthal orientation means is normally required to keep a conventional upwind rotor properly aimed into the wind. This directional orientation means normally comprises either downwind fluid reaction surfaces, such as a tail fin, or an active directional control mechanism. Either solution adds extra cost, weight, wind resistance, and complication to an installation, while not otherwise contributing to power generation.
6. Safety Issues:
It is possible for virtually any wind turbine to undergo structural failure at some point in its service life. With tip speeds often-exceeding 150 mph (xcx9cmach 0.2), the ponderously large blades of conventional wind turbines store a tremendous amount of kinetic energy, and are known to be very dangerous if broken or detached, even in home installations. These huge rotor blades, (with a mass proportional to the diameter cubed, even though the power collected is only proportional to the diameter squared) often require a heavyduty crane to be lifted into place. On the average one person dies every year in such operations.
7. Vibration issues: Prior art turbines are known to transmit low frequency vibration to structures upon which they are mounted, often making rooftop mounting inadvisable.
8. Noise issues:
Conventional windmills with a single rotor often produce noise in high winds, which may be objectional in residential areas.
The invention presented in U.S. patent application Ser. No. 09/881,511 by this inventor addresses and solves these drawbacks of prior art. That invention as disclosed, in several of its main embodiments, places a multiplicity of substantially conventional horizontal axis rotors at spaced intervals along a single, semi-flexible tower/driveshaft. This tower/driveshaft protrudes from its base into the windstream, naturally bending downwind to properly orient the rotors for power generation. The entire structure is caused to spin along its longitudinal axis, transmitting useful power to the base using only a single moving part. The coupling of multiple rotors achieves a high rate of rotation, with more total power than single rotor designs. This high power is delivered at a fast rate of rotation, and therefore at low torque, allowing the shaft to be less substantial than it would need to be to deliver the same power at a slower rate of rotation. The present invention addresses two challenges inherent in this previous design, as disclosed by this inventor:
1. Stress on the Tower/Driveshaft:
In certain embodiments, the tower/driveshaft of this previously disclosed invention protrudes substantially perpendicular to the wind direction, and must then bend downwind to properly orient the rotors. This tower/driveshaft must support the weight of the rotors, resist the force of the wind thereupon, and transmit the rotational torque generated thereby to the base, all while spinning about its longitudinal axis, while in a bent configuration. This transmission of torque by a shaft that is both spinning and bent under load is very stressful to the shaft.
2. Stress on the Cantilevered Bearing Means:
The downwind forces exerted on the multiple rotors and their supporting tower/driveshaft, by the wind, and by their weight, as transmitted through the leverage afforded by the length of the tower/driveshaft, result in large radial loading upon the bearings of the cantilevered bearing means at the base.
The present invention is a modified, more balanced version of the invention disclosed in U.S. patent application Ser. No. 09/881,511. In that disclosure, the rotating tower/driveshaft served to elevate the rotors, and bent downwind to properly orient them. The present invention retains the support, such as a stationary tower, of a conventional wind turbine, while nonetheless preserving several of the advantages of the embodiments disclosed in U.S. patent application Ser. No. 09/881,511.
In this new, more balanced version, the driveshaft extends both forward, substantially into the direction of the wind, as well as backward, or substantially downwind. This more balanced configuration involves less leverage, and results in less stress on the cantilevered bearing means, less stress on the shaft, as well as requiring less bending of the shaft. The entire assembly is mounted on a conventional support means, such as a tower, building, tree, pole, or other elevating structure. Since the shaft protrudes in two directions from the cantilevered bearing means, the stress on the shaft is automatically cut at least in half. Since no part of the driveshaft is acting as the tower, leverage and stresses on the shaft are further reduced. And since the length of shaft presented is more parallel to the wind, bending stresses on the shaft are even further dramatically reduced.
The direction of projection of the shaft, while having a major component substantially parallel to the wind, may be offset from the actual wind direction in an amount sufficient to allow an intermixture of fresh, undisturbed wind into the disk swept by each succeeding rotor, so that each rotor may effectively harness wind energy without undue disturbance from upwind rotors, and substantially contribute toward the overall rotation of the shaft.
Some advantages of the embodiments of the previously disclosed invention, in common with the invention disclosed herein are:
1. Lighter rotor weight:
A multiplicity of smaller rotors weighs less than a single larger rotor sweeping an equivalent total area. This is because the mass of a rotor is proportional to the third power of the diameter (diameter cubed), while the area swept is only proportional to the second power of the diameter, (diameter squared). The larger the rotor, the less wind it can capture relative to its mass.
2. Faster rotation:
For a given rotor type, in a given wind speed, the tip speed is basically some multiple of the wind speed, independent of rotor diameter. Therefore, smaller rotors rotate at a faster rate (rpm) than larger rotors. The multiplicity of smaller rotors of the present invention has a faster rate of rotation (rpm) than a single larger rotor of equivalent swept area. Since electrical generators perform best at such a relatively high rate of rotation (rpm), the present invention more closely matches the desired rotation rate (rpm) of current electrical generating equipment. This means that a gearbox is either not needed, or, if needed, may be less substantial than would be the case with a single, large, slowly spinning rotor with its commensurate high torque. One version of the present invention even takes advantage of counter-rotating sets of rotors, and their differential relative rate of rotation, which essentially doubles the effective rate of rotation.
3. Lighter Duty Drivetrain:
A faster-rotating driveshaft can transmit the same power at less torque than a more slowly rotating driveshaft. Since the present invention rotates faster, torques are lower, requiring a less substantial drivetrain. This lowers cost, as well as further lowering overall weight.
4. Self-Aiming Behavior:
The previously disclosed invention was a downwind machine.
The present invention, can easily be configured as a self-orienting, semi-downwind machine, either by having more rotors downwind than upwind of an azimuthal pivot point, or by giving the downwind rotors more leverage, spaced further from the pivot point, so that the windmill naturally points into the wind, in the fashion of a weathervane.
5. Safety:
Multiple smaller rotors store less kinetic energy than equivalent larger ones. This translates to less danger should mechanical failure occur.
6. Vibration:
The low frequency vibrations associated with larger rotors are reduced or eliminated with multiple smaller rotors, making rooftop installations more practical.
7. Noise:
The multiple small rotors will have different noise characteristics, in high winds, than single larger ones, and may therefore be less objectionable to nearby residents. Some advantages that the embodiments disclosed herein have over the previously disclosed embodiments of U.S. patent application No. 09/881,511 are:
1. Since the shaft protrudes in two directions from the cantilevered bearing means, the stress on the shaft is automatically cut at least in half in the present invention.
2. Since no part of the driveshaft is acting as the tower, the overall length of the shaft, as well as leverage and stresses on the shaft are further reduced.
3. Since the length of shaft presented is more parallel to the wind, bending stresses on the shaft are even further dramatically reduced, because the wind has less leverage.
4. The radial loads on the bearings are dramatically reduced, since the windmill is largely balanced about the bearings, since the driveshaft protrudes therefrom in both directions.
Some disadvantages associated with this new design are:
1. We give up the xe2x80x9csingle moving partxe2x80x9d of the previous design, assuming that the present invention is mounted with directional freedom.
2. We must use a conventional elevated mount, such as a tower.
3. We give up having the load located at ground level.
4. We give up having a non-rotating loadxe2x80x94that is, in this case, the orientation of the load changes depending on wind direction, meaning that a simple power cable from the load to the ground could eventually become twisted too far in one direction, so accommodations must be made for that fact; a means, such as slip rings, or a cable that can be unplugged, untwisted, then re-plugged in, must be used for transmitting power to the ground, unless the installation happens to be in a location where the wind never changes direction, or the installation is hard-mounted in a certain direction, or an active aiming mechanism is used, so that the cable may be untwisted at will automatically by rotating the orientation of the assembly on the tower.
Nevertheless, as every design involves tradeoffs, for many applications the embodiments of the present invention do indeed exhibit superior cost and performance characteristics overall.